I grew up on the Sunny 16 system and still at 63 use it before turning on the meter if I have it with me. With practice, you eye and mind can follow light and quickly judge the adjustments starting with the rule but I seriously doubt many have made a project or set a goal of learning the system. I used to teach it to my 1st year photogrpahy students who generally as a group quickly grasped the concepts as thye had not be trained to rely on a meter as of yet.
If you want a good, actually excellent, system chart go find an older Rolleiflex that has the latitude and time adjustments on the chart for the system. In testing it I found it was a good refinement of the general rule but possibly because of changing atmospheric conditions and the ozone layer just provided a better jumoing off place to start.
I think the largest issue with the system is one similar to using a meter which is what is the true speed of the film. It seems to me at least that with older film the listed speed seemed closer to its ideal; maybe a reult of more subjective testing back them. If the listed film speed is a stop or more off from the ideal you have to adjust accordingly and why you may have some error introduced with the system. An iso 100 film better rated at iso 50 will throw off the system by an entire stop and it is not unusual for todays' films to be that far off. Maybe the makers are simlply relying on the film latitude to make up the difference and they "need" a certain listed film speed for marketing reasons.
I seem to remember Fuji (I remember it being Fuji) at 1 time published a modified Sunny 16 guide in one of its color film cartons that was a Sunny 11 syetem. A fellow photogrpaher who knew film better than I told me it was because the listed speed was not the same as the optimal speed and hence the maker modded the system on the box to advise buyers to set their metering to the preferred optimal speed. I guess a way to get a specific film speed to market for the marketing guys while telling those smart enough what the photographers what it really should be shot at.